1. Field of the Invention
This invention pertains to the general field of phase detectors for use with phase locked loops and in particular pertains to the use of a phase detector in a circuit where input pulses may be missing.
2. Description of the Prior Art
The initial acquisition of a phase locked loop when used for timing or carrier extraction is a significant practical problem since the narrow bandwidth generally required for jitter requirements severely restricts the pulling range. In the prior art many methods have been used to effect the acquisition. These methods are summarized in an article entitled "Frequency Detectors for PLL Acquisition in Timing and Carrier Recovery", David G. Messerschmitt, IEEE Transactions on Communications, Vol. -27, No. 9, September 1979. This article summarizes a significant relevant portion of the prior art. U.S. Pat. Nos. 4,308,505; 3,458,823; 4,363,004 and this article all show the same arrangement of a phase detector, a frequency detector, a filter and a voltage controlled oscillator as used in the type of circuit for which the present invention would be used, except that the input to the frequency detector in the present invention is from a crystal oscillator, while this input is the regular data signal in these four references. This difference of input results in a completely different operation. The circuit of the present invention works on different principles than those disclosed in the prior art.